I’ve been hearing more and more about the use of CVT transmissions in some of the newer cars. Can you shed some light on what these are all about?

Laurence Waltrip,

San Rafael, Calif.

The continuously variable transmission began with the first gasoline-powered automobile built by Daimler and Benz in 1886. A rubber belt stretched between two variable-diameter pulleys allowed an infinite change in drive ratios to best suit the engine’s torque curve.

With the experience gained by use in more than 1 million small cars , CVTs are now becoming viable for larger vehicles.

Most CVT transmissions use a VDT (Van Doorne’s Transmisie) multisegment steel belt, which is pushed, rather than pulled, through the pulleys, in order to handle large torque loads. There are a variety of methods used to allow the vehicle to come to a stop with the engine running: a torque converter or a magnetic or hydraulic clutch. Electronic control of pulley diameter and side pressure gives the CVT the efficiency of a manual transmission with the convenience of an automatic, along with less weight and greater simplicity.

I recently took a demonstration drive in a 2005 Passat TDI sedan. I liked the car in every respect except one: There is no dipstick to check the transmission fluid. The dealer service adviser said a diagnostic computer and special tools are required to check and top off the fluid. I think the lack of a simple dipstick could lead to expensive service or repairs.

Bob Hall, Groton, Conn.

A growing number of automakers have eliminated the automatic-transmission dipstick, placing this component in the same company as a manual transmission, 4×4 transfer case or rear differential.

The reasoning varies: Fluid replacement or additions are no longer deemed necessary, to lessen the chance of contamination or improper fluid being added, or perhaps to simply save a buck or two. Checking the fluid level generally requires using a scan tool to ensure the transmission is at the specified temperature, then a fitting is opened on the side of the transmission. The presence or lack of fluid at this location indicates proper fluid level. Adding fluid is more of a challenge than before as there is no longer a filler tube in which to pour it.

Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif.

E-mail him at under-the-hoodjuno.com or write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. He cannot make personal replies.)

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AP-NY-01-28-05 0616EST